See bleapputils.h:
/// This function gives a microsecond clock; this is a 16-bit counter,
/// counting up.
/// \return The 16-bit counter value, which rolls over every 65.5 ms.
UINT16 bleapputils_currentTimeUs(void);
EDIT:
Sorry, the earlier method will not be accurate on the 2073* family of chips. Use these instead (all based on the Bluetooth clock, with min resolution of 312.5uS; there is no timer with a shorter resolution accessible from the application):
/// This function returns the current native Bluetooth clock; this
/// counter is 28 bits, and ticks every 312.5 us and is adjusted for
/// drift through sleep, etc. To compute differences and times elapsed,
/// use bleapputils_diffNativeBtClks() or bleapputils_BtClksSince().
/// \return the counter value.
/// \return The current BT clock value.
UINT32 bleapputils_currentNativeBtClk(void);
/// This function computes the signed difference between two BT
/// clock instants. The general "garbage-in garbage-out" principle
/// applies, so clocks must be from the same piconet, must be valid when
/// they are taken, etc. BT clock is 28 bits only and cannot be relied upon when
/// timings in the hours are desired.
/// \param from is the "from" time.
/// \param to is the "to" time.
/// \return the signed difference (to - from); positive if "to" is after "from",
/// negative otherwise.
INT32 bleapputils_diffBtClks(UINT32 from, UINT32 to);
/// This function computes the time elapsed since "before", in BT
/// clocks. This functions handles rollovers. Clock resets will cause
/// a large value to be returned.
/// \param before is the previous counter value, as was returned by
/// hiddcfa_currentNativeBtClk().
/// \return the time elapsed, in BT clocks (312.5 us).
UINT32 bleapputils_BtClksSince(UINT32 before);
